Selective hydrogenation of styrene to ethylbenzene

ABSTRACT

A feedstream comprising paraxylene and styrene is contacted, in the presence of hydrogen, with a catalyst comprising at least one metal, selected from one or more metals selected from Groups 8-10.

PRIORITY

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 14/165,255, filed Jan. 27, 2014, and U.S.Provisional Application No. 61/761,402, filed Feb. 6, 2013, thedisclosures of which are incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to the selective hydrogenation of styrene toethylbenzene and more particularly to the removal of small quantities ofstyrene present in the product stream of a method of making paraxyleneselectively by the alkylation of aromatic species with an alkylatingagent over a solid catalyst.

BACKGROUND OF THE INVENTION

It is well-known to manufacture xylenes by the alkylation of tolueneand/or benzene with methanol, and in particular to selectively makeparaxylene (PX) product using zeolite catalyst. See, for instance, U.S.Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690;5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879;6,504,072; 6,506,954; 6,538,167; and 6,642,426. See also more recentlyU.S. application Ser. No. 13/557,605, and references cited therein.Paraxylene selectivity is highly sought after because of the economicimportance of paraxylene relative to meta- and orthoxylene. Althougheach of the xylene isomers have important and well-known end uses,paraxylene is currently the most economically valuable, serving as anintermediate in such important and diverse end uses as bottle plasticand polyester fibers.

One of the problems with xylenes streams produced by alkylating aromaticspecies such as benzene and/or toluene with alkylating agents such asmethanol and/or dimethyl ether over solid catalysts, such as in theaforementioned processes, is the product stream may contain styreneimpurities. This has recently been observed and set forth in ProvisionalPatent Applications 61/711,341 and 61/681,486. Styrene impurities cancause operability problems for downstream process, for exampleparaxylene recovery by adsorptive separation processes, e.g., Parex™Process or Eluxyl™ Process, as well as other processes used to takeparaxylene to end products, such as in processes used to make purifiedterephthalic acid/anhydride and subsequent steps to making fibers orbottle plastic therefrom.

One method of removing styrene is to convert said species toethylbenzene by selective hydrogenation. Several characteristics ofpurifying xylenes containing styrene impurities make selectivehydrogenation of styrene challenging. It is highly desirable to minimizethe ring saturation reactions since separation of dimethylcyclohexane,ethylcyclohexane, and/or other saturated C8 hydrocarbons from xylenes isdifficult. Another potential challenge is that the desired product,paraxylene, is present at higher-than-equilibrium concentration. Thecatalyst used to hydrogenate styrene must therefore show minimal xylenesisomerization activity.

It is known to hydrogenate certain aromatic species in paraxyleneenriched streams. See, for instance, U.S. application Ser. Nos.61/604,926, 61/496,262, 13/303,855, and 13/449,758.

The present inventors have surprisingly discovered a method ofselectively hydrogenating styrene impurities present in a xylenes streamnearly stoichiometrically using a catalyst comprising at least one metalselected Groups 8-10 of the Periodic Table, optionally furthercomprising promoters and/or supports.

SUMMARY OF THE INVENTION

The invention is directed to a method of hydrogenating styreneimpurities present in a xylenes stream using a catalyst comprising M,wherein M is selected from Group 8-10 metals, preferably Pd, Co, Ni, Ru,and mixtures thereof.

In embodiments, promoters such as Ag, Au, In, K and other alkali metals,Ca and other alkaline earth metals, and mixtures thereof, can bepresent.

In embodiments, a support such as Al₂O₃, carbon, SiO₂, TiO₂, andmixtures thereof, may be present.

It is an object of the invention to eliminate styrene fromparaxylene-containing feedstreams prior to processes that would haveoperability problems with even small quantities of styrene, e.g.,catalytic processes, absorption processes, and the like, that aresensitive to the presence of vinyl moiety, while having little or noeffect, e.g., ring saturation and/or isomerization, on paraxyleneitself.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

DETAILED DESCRIPTION

According to the invention, a feedstream comprising paraxylene andstyrene is contacted in the presence of hydrogen with a catalystcomprising one or more metals selected from Groups 8-10 of the PeriodicTable, preferably Pd, Co, Ni, Ru, and mixtures thereof.

In embodiments, promoters such as Ag, Au, In, K and other alkali metals(Group 1 metals), Ca and other alkaline earth metals (Group 2 metals),and mixtures thereof, can be present.

In embodiments, a support such as carbon, alumina (Al₂O₃), silica(SiO₂), titania (TiO₂) and mixtures thereof, may be present.

In preferred embodiments the amount of paraxylene in the feedstream fromthe alkylation reaction is enriched from equilibrium concentration,i.e., greater than about 24 wt %, and in preferred embodiments is in theamount of from 70 wt % to 98 wt %, or 75 wt % to 92 wt %, or 80 wt % to89 wt %.

In embodiments the amount of styrene in the feedsteam from thealkylation reactor is in the range of from 0.01 wt % to 2.00 wt %, or0.05 wt % to 1.00 wt %, or 0.08 wt % to 0.50 wt %.

In preferred embodiments, less than 0.2 wt % of the para-xylene isisomerized in the step of contacting one or more metals selected fromGroups 8-10.

In preferred embodiments, there is no detectable ethylcyclohexane ordimethylcyclohexane in the product (based on gas chromatographicanalysis) in the step of contacting one or more metals selected fromGroups 8-10.

The preferred catalyst for the alkylation reaction comprises ZSM-5,which has been severely steamed at a temperature of at least 950° C. inthe presence of at least one oxide modifier, preferably includingphosphorus, to control reduction of the micropore volume of the materialduring the steaming step. See U.S. Pat. Nos. 6,423,879 and 6,504,072.The preferred alkylating agent is methanol or dimethyl ether and thepreferred aromatic species alkylated is benzene and/or toluene.

The invention will be better understood by reference to the followingexample, which will be understood by those of ordinary skill in the artto be representative and not limiting thereof.

To illustrate the effectiveness of Pd catalysts in hydrogenation ofstyrene by-product found in xylene product stream from an alkylationreactor such as a fluid bed reactor having staged baffles, such asdescribed in U.S. application Ser. No. 13/557,605, and such as accordingto the process described in U.S. Provisional Application 61/681,486, arun was made with Axens™ LD269 catalyst and a feed consisted of 0.10%styrene, 0.62% ethylbenzene, 78.72% para-xylene, 14.66% meta-xylene, and5.58% ortho-xylene. Axens™ LD269 is a commercially available catalystused for butene hydrogenation. It is a Pd/Al₂0₃ catalyst based on theinformation available in the public domain. The test was carried out ina down flow micro-lab unit at 100° C., 165 psig, 5 hr⁻¹ WHSV, and 1/1H₂/HC molar ratio. Key results from the test are summarized in the tablebelow.

TABLE 1 HIGH CONVERSION AND HIGH SELECTIVITY ACHIEVED IN HYDROGENATIONOF STYRENE (100° C., 165 psig, 1/1 H₂/HC molar ratio, 5 hr⁻¹ WHSV)Components, wt % Feed Product Toluene 0.13 0.12 Ethylbenzene 0.62 0.72Para-Xylene 78.72 78.63 Meta-Xylene 14.66 14.71 Ortho-Xylene 5.58 5.60Styrene 0.10 0.00 Phenol 0.0012 0.0011 Styrene Conversion NA 100 StyreneSelectivity to EB NA 100

As can be seen from the table, Axens™ LD 269 catalyst is very effectivein hydrogenating styrene present in the mixed xylene feed toethylbenzene. No measurable styrene was found in the product by gaschromatography, equipped with wax column. The detection limit of the GCis 5 ppm. This suggests that conversion was nearly 100% if not 100% at100° C., 165 psig, and 5 hr⁻¹ WHSV. Selectivity was also nearly 100% ifnot 100% since all the styrene which was converted can be accounted byfor the increase in ethylbenzene. No ethylcyclohexane ordimethylcyclohexane was detected in the product. The detection limit ofthe GC for measuring ethylbenzene, ethylcyclohexane anddimethylcyclohexane was also 5 ppm.

In addition to being highly active and selective in hydrogenation ofstyrene to ethylbenzene, Pd/Al₂0₃ catalysts such as Axens™ LD269 werealso found to have minimal activity in isomerization of para-xylene toortho-xylene or para-xylene. As can be seen from the table, only ˜0.1 wt% para-xylene was isomerized to either ortho-xylene or meta-xylene(based on the total amount of para-xylene). The relatively completeconversion of styrene and lack of typical isomerization products bycontact of a mixed xylene feed was highly surprising.

The invention has been described above with specificity however it willbe understood by one of ordinary skill in the art in possession of thepresent disclosure that the invention may be practiced other than asspecifically set forth herein, such as in fixed bed reactors, moving bedreactors, and the like.

All patents and patent applications, test procedures (such as ASTMmethods, UL methods, and the like), and other documents cited herein arefully incorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. Trade names used herein are indicated by a ™symbol or ® symbol, indicating that the names may be protected bycertain trademark rights, e.g., they may be registered trademarks invarious jurisdictions. When numerical lower limits and numerical upperlimits are listed herein, ranges from any lower limit to any upper limitare contemplated.

What is claimed is:
 1. A process comprising: (i) contacting an aromatichydrocarbon feedstream comprising benzene and/or toluene with methanoland/or dimethyl ether in the presence of a phosphorus-modified ZSM-5alkylation catalyst, which has been steamed at a temperature of at least950° C., to obtain a first product stream comprising greater than about24 wt % para-xylene and styrene; and (ii) contacting said first productstream comprising greater than about 24 wt % para-xylene and styrenewith a hydrogenation catalyst comprising at least one metal selectedfrom Groups 8-10 of the Periodic Table, in the presence of hydrogen, toobtain a second product stream comprising a reduced amount of styrenewhen compared with said first product stream, wherein less than 0.2 wt %of paraxylene is isomerized by contact with the hydrogenation catalyst.2. The process of claim 1, wherein said second product stream is furthercharacterized as having no detectable levels of styrene,ethylcyclohexane and dimethylcyclohexane, as measured by gaschromatography.
 3. The process of claim 1, wherein said hydrogenationcatalyst further comprises a support selected from Al₂O₃, carbon, SiO₂,TiO₂, and mixtures thereof.
 4. The process of claim 1, wherein saidmetal is selected from Pd, Co, Ni, Ru, and mixtures thereof.
 5. Theprocess of claim 1, wherein said hydrogenation catalyst furthercomprises at least one metal or metal compound selected from Ag, Au, In,alkali metals, and alkaline earth metals.
 6. The process of claim 1,wherein said second product stream contains an increased amount ofethylbenzene relative to said feedstream.
 7. The process of claim 6,wherein said increased amount of ethylbenzene is equal to the amount ofstyrene in said first product stream as measured by gas chromatography.8. The process of claim 1, wherein the amount of paraxylene in the firstproduct stream comprises at least 70 wt % based on the total xyleneconcentration.
 9. The process of claim 1, wherein the amount of styrenein the first product stream comprises about 0.01 wt % to 2.00 wt %.